The signal transmitted by a GNSS satellite may be affected by random errors which lead to a distortion of the signal so that it does not conform to the specifications of the system. Such errors concern, for example, a random modification of the period of the spreading codes which results in a stationarity break in the signal or even the presence of ripples which modify the square shape, characteristic of a signal modulated by a spreading code, expected at the output of the signal generator.
Such errors are linked to the implementation of the transmitter on board the satellite and in particular to the defects in the equipment items which are not perfect.
The measurements performed on a non-compliant GNSS signal affected by such defects are subject to errors and will result in an incorrect positioning of the GNSS receiver which uses this signal. It is therefore important to be able to detect such errors in order to alert the receiver to the non-conformity of the signal that it is using and to avoid positioning or navigation errors which can have critical consequences.
Hereinafter, the expression “evil waveform” signals will be used to designate GNSS signals affected by errors or distortions which are the consequence of defects in the hardware equipment items on board the satellite.
The known solutions that are used to detect “evil waveform” signals are often based on the use of the correlation function at the output of the integrators of the GNSS receiver. The main drawback of this technique is that it does not differentiate the errors arising from the imperfections of the transmitter on board the satellite and those arising from the multiple paths linked to the reflections of the signal on the different layers of the ionosphere. Also, these techniques are highly sensitive to the ambient noise. The errors associated with the transmission of the signal can be corrected by appropriate processing functions and must be dissociated from the errors introduced from the generation of the signal.